You can add one video track per audio montage.
Procedure
In the Audio Montage window, do one of the following:
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Click + at the top of the track control area and select Video Track.
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Right-click the track control area to open the Track pop-up menu and select .
Result
By default, the new track is added below the active track. If you want to place it above the active track, press Ctrl/Cmd when adding the new track.